Chitosan, which is a β1,4-linked glucosamine polymer, is known to provide antimicrobial activity useful in a wide range of applications including in food preparation and packaging, in personal hygiene such as in garments and personal care articles, and in locations with high potential for microbial contamination such as bathrooms and hospitals. In addition, oligosaccharides of chitosan derivatives were found to have anti-inflamatory properties providing potential use as a pharmaceutical, dietary supplement, or cosmetic component for treatment of inflammation (WO 03026677). Polygalactosamines with alpha linkages are found in nature and β-galactosamines in an acylated form are found as structural components of chondrointin sulfate and dermatan sulfate, compounds that are a part of the proteoglycan structure found in cartilage. Low molecular weight polygalactosamines may have chondroprotective effects and/or other important biological properties, and thus may be useful pharmaceutically.
Chitosan is the commonly used name for poly-[1-4]-β-D-glucosamine. Chitosan is chemically derived from chitin, which is a poly-[1-4]-β-N-acetyl-D-glucosamine, and which, in turn, is derived from the cell walls of fungi, the shells of insects and, especially, crustaceans. Chitin is treated with strong alkalis to remove acetyl groups producing chitosan. Depending on the specific treatment of chitin, chitosan can vary in the degree of deacetylation. Subsequent treatment with mineral acids or enzymes is used to break down the natural chitosan polymers (up to around 1.2 megaDaltons in size) into shorter β1,4-linked 2-amino-2-deoxy-glucopyranosyl polymers. Chitosan preparations that are obtained in this manner, and are commercially available at low cost (from, for example, Primex Corporation (Norway), Biopolymer Engineering, Inc. (St. Paul, Minn.), Biopolymer Technologies, Inc. (Westborough, Mass.), and CarboMer, Inc. (Westborough, Mass.)), generally contain polymers that include a range of sizes, including polymers that are larger than 10,000 Daltons in size and are insoluble in aqueous media, but can be dissolved by converting the glucosamine residue to its acid salt. This limits their application, especially in the food industry, since the salts tend to produce an undesired taste.
Commercial scale preparations of predominantly single chain length low molecular weight polyglucosamines derived from chitosan are generally not available due to the difficulty of purification. An alternative source of these polymers would be through chemical synthesis, yet there has been little success. Forming linkages of glucosamine and galactosamine at the 3-, 4-, and 6-hydroxyls can be problematic due to low reactivity, with the 4-hydroxyl groups of glucosamine or galactosamine units having particularly low reactivity. Kanie et al. (J. Am. Chem. Soc., 1994, 116, 12073-12074) had limited success in the synthesis of a precursor of a gluocsamine heptamer using block synthesis of fragments containing orthogonal protecting groups. US 2004/0019198 discloses a process in which a glycosidic bond is formed using a thioglycoside that is activated by an N,N-dialkylsulfinamide and trifluoromethanesulfonic anhydride. Using the disclosed method, polymer-supported synthesis of β-mannopyranoside-type glycosidic bonds is achieved, producing various β-mannosides.
Thioglycosides are known to be shelf stable monomers for use in the synthesis of oligosaccharides (Fugedi et al., Glycoconjugate Journal, 1987, 4:97-108). However, thioglycosides generally have to be activated with strong electrophiles prior to coupling to sugar hydroxyl groups. Thioglycoside activation procedures are known and are described in US 20040019198, but application of the activated thioglycosides for the efficient synthesis of multi-gram quantities of β-linked low molecular weight polymers of galactosamine and glucosamine is lacking.
Thus there is a need for processes for economical large-scale synthesis of single-species enriched free β-linked low molecular weight polymers of galactosamine and glucosamine. Such polymers are useful in antimicrobial, anti-inflammatory, and/or other applications.